Dynamin, a 100 kDa GTPase, is believed to be involved in the constriction of clathrin coated pits and may cause fission of clathrin coated vesicles during receptor mediated endocytosis and during membrane retrieval in nerve terminals. It has been shown that purified dynamin incubated under low salt conditions forms rings and spirals which, in dimension and appearance, resemble the dense material occasionally observed at the necks of coated pits. We have shown that purified dynamin can also be induced to form spirals under physiological salt conditions by incubating it with GDP and (-phosphate analogues (BeF or AIF) or by dialyzing it into GTP(S. This demonstrates that the polymerized state of dynamin is markedly stabilized by sustaining its GTP or GDP/Pi-bound conformation. Moreover, spirals still form when dynamin is proteolyzed to either a predominant 80 kDa species that lacks the C-terminus, or when it is proteolyzed into two smaller fragments, a 55 kDa species originating from the N-terminal half of the protein and a 30 kDa central domain lacking the C-terminus. The formation of dynamin spirals was quantified by a sedimentation assay and imaged by electron microscopy using either negative staining techniques. Currently we are purifying dynamin fragments to determine which regions of dynamin are responsible for assembly. Our new ability to stabilize dynamin spirals in physiological salt solution strengthens the idea that in vivo, even in the absence of other protein cofactors, dynamin can assemble into spirals around the necks of coated pits and thereby promote vesicle fission. We have also shown by negative stain electron microscopy that purified recombinant dynamin is able to bind to lipid vesicles to form helical tubes which are similar in dimensions to dense material seen around the necks of clathrin-coated pits. These tubes diffract to approximately 30 angstroms and will be used to determine the three dimensional structure of dynamin by helical reconstruction methods. In addition, by using both light and electron microscopy, we have evidence that GTP hydrolysis causes a major structural change in the dynamin/lipid tubes which may represent the process which occurs upon vesicle scission. When GTP is added to dynamin tubes, they appear to twist and form small vesicles. These results provide strong evidence that dynamin is the structural component necessary for the formation of the constricted necks of coated pits, and support the hypothesis that dynamin is the force-generating molecule responsible for membrane fission.